On Dec. 7, 1971, two patents issued in which one of the coinventors hereof, Leo Esaki, was also a coinventor, such patents being U.S. Pat. Nos. 3,626,257 and 3,626,328 entitled "Semiconductor Device with Superlattice Region" and "Semiconductor Bulk Oscillator", respectively, both of which are incorporated herein by reference. These patents teach how to produce semiconductor devices with superlattice regions, the electrical properties thereof, and point out the prior state of the art with respect to theoretical considerations involving wave propagation in periodic structures, and the bulk negative resistance devices resulting therefrom.
Prior to our invention, Leo Esaki and Raphael Tsu, realized that the normal conduction band energy levels of a superlattice region are modified to produce supplemental allowed levels separated by infrared frequencies in superlattice structures. They taught that such devices could be useful as sources of infrared radiation and as laser amplifiers and oscillators.
Radiation sensitive devices in the near IR visible region are presently available which are responsive to such radiation. These are commonly known as solar cells. Thus far, no radiation responsive devices in the far IR region have been developed which have sensitivity comparable to such known solar cells.